Rollup merge of rust-lang#128309 - kmicklas:btreeset-cursor, r=Amanieu

Implement cursors for `BTreeSet`

Tracking issue: rust-lang#107540

This is a straightforward wrapping of the map API, except that map's `CursorMut` does not make sense, because there is no value to mutate. Hence, map's `CursorMutKey` is wrapped here as just `CursorMut`, since it's unambiguous for sets and we don't normally speak of "keys". On the other hand, I can see some potential for confusion with `CursorMut` meaning different things in each module. I'm happy to take suggestions to improve that.

r? ````@Amanieu````